Larks

Stealing upon larks

When Bloom hurries out of the Telegraph office on Middle Abbey Street hoping to waylay Alexander Keyes about the renewal of an ad, Professor MacHugh and Lenehan are amused to see a file of newsboys following him:

Taking off his flat spaugs and the walk. Small nines. Steal upon larks. (U 7.448-9)

As "Small nines" was a late insertion which refers to Bloom's shoe size, it seems likely that "Steal upon larks" describes Bloom's rather than the boys' walk.1
This last expression is documented and explained in Pádraic Colum's My Irish Year, published in 1912.

A slow and cautious character she [an old woman] called ‘Martin-steal-upon-larks’. (p. 86)

Though Bloom may not be exactly slow here, his step obviously looks sneaky to the boys and to the observers in the office. We are reminded of the impression Bloom's walk makes on Stephen when he passes through the library gate before him: "step of a pard" (9.1214) .